4 December 2016 | Star Trail

A star trail is a type of photograph that utilizes long-exposure times to capture the apparent motion of stars in the night sky due to the rotation of the Earth. A star trail photograph shows individual stars as streaks across the image, with longer exposures resulting in longer streaks. Typical exposure times for a star trail range from 15 minutes to several hours, requiring a ‘bulb’ setting on the camera to open the shutter for a longer period than is normal.

Star trail photographs are possible because of the rotation of the Earth on its axis. The apparent motion of the stars is recorded as streaks on the film or detector. For observers in the northern hemisphere, aiming the camera towards the north creates an image with concentric circular streaks centered on the north celestial pole (very close to Polaris). For observers located in the southern hemisphere, this same effect is achieved by aiming the camera south. In this case, the streaks are centered on the south celestial pole. Aiming the camera towards the east or west creates straight-line streaks that are angled with respect to the horizon. The size of the angle depends on the photographer’s latitude.

Star trail photographs can be used by astronomers to determine the quality of a location for telescope observations. Star trail observations of Polaris have been used to measure the quality of seeing in the atmosphere, and the vibrations in telescope mounting systems. The first recorded suggestion of this technique is from E.S. Skinner’s 1931 book A Manual of Celestial Photography.